1. Field of the Invention
The present invention relates to a four wheel drive vehicle, particularly an agricultural tractor, comprising a front wheel drive transmission including a first clutch engageable to establish a first four wheel drive mode in which an average peripheral speed of right and left front wheels is equal or approximately equal to an average peripheral speed of right and left rear wheels, and a second, multidisk clutch engageable to establish a second four wheel drive mode in which the average peripheral speed of the right and left front wheels is higher than the average peripheral speed of the right and left rear wheels.
2. Description of the Prior Art
A four wheel drive tractor as described above is known, for example from U.S. Pat. No. 4,574,910. In order to effect a smooth front wheel change speed operation and to simplify an interlocking structure in the case of automatic change speed, the tractor disclosed in this U.S. patent comprises a front wheel drive transmission including a first and a second clutches which are driven by two hydraulically operable pistons, respectively. Generally, where the clutches are driven by means of oil pressure, power for driving a hydraulic pump to supply the pressure oil to these pistons is taken out at a position downstream of a main clutch operable to establish and break transmission of engine power to a transmission case.
Therefore, when the tractor is stopped by applying brakes to the rear wheels and the main clutch is disengaged to avoid engine stopping, the hydraulic pump stops operating and the front wheel drive transmission assumes neutral whereupon the rear and front wheels are brought out of operative connection which frees the front wheels from the braking action to the detriment of braking effect. When the hydraulic system fails to function properly, the front wheel drive transmission assumes neutral and four wheel drive becomes impossible. Further, pressure oil must be supplied constantly to maintain clutch engagement, which tends to raise oil temperature.
This type of agricultural tractor makes a small sharp turn by accelerating the front wheels, and for this purpose the front wheel drive transmission is switched to the second four wheel drive mode in response to movements of control devices of the tractor such as a steering wheel, a side brake pedal and a change speed lever.
Generally, in a model having one of the control devices, for example the steering wheel, and the front wheel drive transmission interlocked to one another, an electromagnetic control valve is provided to supply and exhaust pressure oil to/from the hydraulic valve in the accelerating drive system and a limit switch or the like is provided to electrically detect an operation of the steering wheel exceeding a predetermined angle and to produce an electric signal for operating the electromagnetic control valve.
The operation of the steering wheel exceeding the predetermined angle is easily detected on the basis of movement of the pitman arm disposed below the tractor chassis. However, this tends to deteriorate the reliability and durability of the limit switch and other electric elements in the case of a working vehicle used to travel on an unleveled ground or a paddy field since the electric elements are subjected to mud and water. It is therefore necessary to provide sufficient shielding to assure their reliability and durability, and this results in increased manufacturing cost.
The clutches of the front wheel drive transmission may be directly and operatively connected to the control device of the tractor through mechanical interlocking means without using hydraulic means at all. However, such a construction inevitably has weakness in operability.